1. Field of the Invention
The present invention relates to a malfunction diagnosis device for diagnosing a malfunction of an exhaust gas purification catalyst for causing ammonia to purify NOx included in exhaust gas discharged from combustion chambers of an internal combustion engine.
2. Description of the Related Art
There is known an exhaust gas purification device (an SCR catalyst) for causing ammonia (NH3) to purify nitrogen oxide (NOx) included in exhaust gas discharged from combustion chambers of an internal combustion engine. There is also known a method for diagnosing a malfunction that a NOx purification rate of the catalyst decreases by using a NOx concentration sensor provided downstream of the catalyst.
For example, a NOx concentration sensor described in JP 9-288084 A can be used for diagnosing a malfunction that a NOx purification rate of the catalyst decreases. This NOx concentration sensor outputs an output value or an electric current depending on amounts of NOx and ammonia included in the exhaust gas.
Accordingly, in a malfunction diagnosis of the SCR catalyst by using the NOx concentration sensor, first, concentrations of NOx and ammonia, which are expected to flow out from the catalyst when the NOx purification rate of the SCR catalyst decreases to a certain constant purification rate, is estimated by calculation. In addition, estimated is an output value of the NOx concentration sensor which is expected to be output from the NOx concentration sensor when exhaust gas including NOx and ammonia having the estimated concentrations, respectively reaches the NOx concentration sensor.
Then, when an actual output value of the NOx concentration sensor becomes equal to or larger than the estimated output value, it is diagnosed that a malfunction occurs in the SCR catalyst.